1,2-dihydro-4-hydroxy-2-oxo-quinoline-3-carboxanilides have been described in the literature since the 1970s (refs 1-4). The most well-known compound in this class, roquinimex (Linomide), was first described by A B Leo as an immuno-stimulating agent (ref 4) but was later also found to have immuno-modulating effects, as well as anti-angiogenetic effects (refs 5a, b). Roquinimex has been claimed beneficial for the treatment of autoimmune diseases, such as rheumatoid arthritis, multiple sclerosis, systemic lupus erythematosus, inflammatory bowel disease, diabetes type 1, and psoriasis, as well as for the treatment of cancer (refs 6a-d, 9d and refs therein).

The compound laquinimod (a 5-Cl, N-Et carboxanilide derivative) has been reported by Active Biotech AB to convey a better therapeutic index compared with roquinimex (refs 7a, b) and is currently in phase III clinical studies for the treatment of multiple sclerosis. Laquinimod has also entered clinical trials in Crohn's disease and SLE. Two other compounds in the same class under clinical evaluation are tasquinimod (prostate cancer) and paquinimod (systemic sclerosis). Recently, a molecular target for laquinimod was identified as S100A9 (ref 8).
Fujisawa has reported on similar compounds with inhibitory activity on nephritis and on B16 melanoma metastases (refs 9a-d). Also the closely related thieno-pyridone analogs have been described as immunomodulating compounds with anti-inflammatory properties (ref 10).
Another closely related compound class are the corresponding N-pyridyl-carboxamide derivatives, which have been reported to have antitubercular activity as well as anti-inflammatory properties (ref 11). However, according to literature (ref 10) these derivatives are less active as immunomodulating agents.
The N-hydrogen 3-carboxanilides (“N—H derivatives”) and the N-alkyl 3-carboxanilides (“N-alkyl derivatives”), respectively, are described in the prior art documents relating to inflammation, immunomodulation, and cancer as a homogenous group of compounds in terms of biological effects. Prior art also teaches that the N-alkyl derivatives are the preferred compound derivatives.

In fact, very few studies (refs 4, 9d) of N-hydrogen derivatives, especially in vivo studies, have been reported. Furthermore, no fundamental biological differences between the N-alkyl derivatives and the N-hydrogen derivatives, respectively, have been described.
However, some chemical properties of the N-hydrogen and the N-alkyl derivatives are different (ref 12). N-Alkyl derivatives adopt a twisted 3D-structure, whereas the N—H derivatives are stabilized by intramolecular hydrogen bonds in a planar structure. The N-alkyl derivatives are more soluble in aqueous media, but also inherently unstable towards nucleophiles, such as amines and alcohols (refs 12, 13).
The N-alkyl derivatives roquinimex (N-Me) and laquinimod (N-Et) have been reported to be metabolized in human microsomes to give the corresponding N-hydrogen derivatives, via N-dealkylation catalyzed mainly by CYP3A4 (refs 14a, b).
bHLH-PAS (basic helix-loop-helix Per-Arnt-Sim) proteins constitute a recently discovered protein family functioning as transcription factors as homo or hetero protein dimers (refs 15a, b). The N-terminal bHLH domain is responsible for DNA binding and contributes to dimerization with other family members. The PAS region (PAS-A and PAS-B) is also involved in protein-protein interactions determining the choice of dimerization partner and the PAS-B domain harbors a potential ligand binding pocket.
The aryl hydrocarbon receptor (AhR or dioxin receptor) and its dimerization partner ARNT (AhR nuclear translocator) were the first mammalian protein members to be identified. AhR is a cytosolic protein in its non-activated form, associated in a protein complex with Hsp90, p23, and XAP2. Upon ligand activation, typically by chlorinated aromatic hydrocarbons like TCDD, the Ahr enters the nucleus and dimerizes with ARNT. The AhR/ARNT dimer recognizes specific xenobiotic response elements (XREs) to regulate TCDD-responsive genes. The ligand binding domain of AhR (AhR-LBD) resides in the PAS-B domain.
Recently, it has been demonstrated that AhR is involved in Th17 and Treg cell development and AhR has been proposed as a unique target for therapeutic immuno-modulation (refs 16a-c). The AhR ligand TCDD was shown to induce development of Treg(FoxP3+) cells, essential for controlling auto-immunity, and to suppress symptoms in the EAE model. In addition, activation of AhR has been shown essential for the generation of IL-10 producing regulatory Tr1 cells (ref 16d), and Ahr ligands have also been proven efficacious in other models of auto-immunity, e.g. diabetes type 1, IBD, and uveitis (refs 16e-h). Apart from controlling autoimmune disorders, AhR activation and Treg cell development have been implicated as a therapeutic strategy for other conditions with an immunological component, such as allergic lung inflammation, food allergy, transplant rejection, bone loss, and type 2 diabetes and other metabolic disorders (refs 17a-e).
Apart from its role as a transcription factor, AhR has been reported to function as a ligand-dependent E3 ubiquitin ligase (ref 18), and ligand-induced degradation of β-catenin has been demonstrated to suppress intestinal cancer in mice (ref 19). In addition, activation of AhR has been implicated to play a protective role in prostate cancer (ref 20).
Other members of the bHLH-PAS family are the HIF-α (hypoxia inducible factor alpha) proteins, which also hetero-dimerize with ARNT. In conditions with normal oxygen levels (normoxia), HIF-α proteins are rapidly degraded by the ubiquitin-proteasome system and they are also inactivated by asparagine hydroxylation. Under hypoxic conditions, however, the proteins are active and upregulate genes as a response to the hypoxic state, e.g. genes for erythropoietin and vascular endothelial growth factor (VEGF). VEGF is essential for blood vessel growth (angiogenesis) and is together with HIF-1a considered as interesting targets for anti-angiogenetic tumour therapy (ref 21). HIF-α proteins can be negatively and indirectly regulated by AhR ligands, which upon binding with AhR reduce the level of the common dimerization partner ARNT. Anti-angiogenetic effects can possibly also be achieved directly by AhR activity via upregulation of thrombospondin-1 (ref 22).